Normally, sodium methacrylate, potassium methacrylate, sodium acrylate, potassium acrylate, or similar metal salts of radical-polymerizable compounds, are obtained by neutralizing aqueous solutions of alkali-metal compounds. However, the problem associated with the aforementioned process consists in that it is extremely difficult to produce metal salts of the radical-polymerizable compounds with low moisture content when they are obtained from aqueous compounds. Many suggestions have been made heretofore to overcome this problem of the prior-art. For example, there were attempts to remove water by subjecting water solutions of radical-polymerizable compounds to azeotropic dehydration in the presence of an organic solvent such as toluene. This method, however, cannot produce metal salts of radical-polymerizable compounds of high purity because the process is accompanied by a polymerization reaction that occurs during azeotropic dehydration. Attempts have been made to suppress this polymerization reaction by adding conventional polymerization inhibitors normally used in conjunction with radical-polymerizable compounds, such as hydroquinone, methoxyhydroquinone, phenothiazine, hindered phenol, or the like. However, since all these compounds are oil-soluble, their solubility in water is low, they cannot completely stop polymerization of radical-polymerizable compounds, and, therefore, are unsuitable for obtaining metal salts of radical-polymerizable compounds of high purity.
On the other hand, known in the art is a method for obtaining moisture-free metal salts of a methacrylic acid, wherein an aqueous component is removed by pulverizing an aqueous solution of a methacrylic-acid metal salt in a hot-air circulation drier (see Japanese Laid-Open Patent Application Publication (hereinafter referred to as Kokai”) 47-031924 and Kokai 49-011820). However, this method is expensive as it requires the use of specific equipment.
Furthermore, a method is know for manufacturing a water-free potassium methacrylate obtained by neutralizing a methacrylic acid in a methanol solution of a potassium methylate (see Kokai 3-209388). However, the potassium methylate used in this process is an expensive reagent, which is not suitable for realization of low-cost production plans.